1. Field of the Invention
The present invention relates to acoustic wave devices and filters, and more particularly, to an acoustic wave device having improved temperature characteristics and a filter using the same.
2. Description of the Related Art
A SAW (Surface Acoustic Wave) device is well known in which comb-like electrodes formed by an IDT (InterDigital Transducer) and reflection electrodes are formed in a surface of a piezoelectric substrate. An acoustic wave is excited by the comb-like electrodes and is propagated on the surface of the piezoelectric substrate. The SAW device may be characterized in that it is compact and light and is capable of attenuating an input signal greatly. Thus, the SAW device is widely used in transmission/reception filters and antenna duplexers employed in cellular phones.
Besides the SAW device, an acoustic boundary wave device has been developed in which the acoustic wave is propagated through an interface between two different media. The acoustic boundary wave device has an advantage that a foreign particle at the interface between the two media does not cause frequency variation and does not increase the electrical loss.
The recent higher performance of the cellular phones requires improvements in the temperature characteristics of the acoustic devices. Japanese Patent No. 3407459 (Document 1) discloses an acoustic wave device (hereinafter referred to as first prior art) designed to improve the temperature characteristic. FIG. 1A is a plan view of an acoustic wave device of the first prior art, and FIG. 1B is a cross-sectional view taken along a line A-A shown in FIG. 1A. Referring to FIG. 1A, comb-like electrodes 14 and reflection electrodes, which may be made of, for example, aluminum, are provided on a piezoelectric substrate made of, for example, lithium tantalate (LiTaO3). For the sake of simplicity, only a small number of fingers of the electrodes 14 are illustrated in FIG. 1A. However, each of the actual electrodes 14 has a large number of fingers. The reflection electrodes 16 are also simplified in FIG. 1A. As shown in FIG. 1B, a silicon oxide (SiO2) film 18 covers the comb-like electrodes 14 and the reflection electrodes 16. In FIG. 1A, the silicon oxide film 18 is omitted for the sake of simplicity. The temperature characteristic of the acoustic wave device thus formed may be improved by setting the thickness of the silicon oxide film 18 equal to or greater than 0.22λ and equal to or less than 0.38λ where λ is the period of the comb-like electrodes 14. The acoustic wave in the above-mentioned structure is called Love wave.
Another acoustic wave device (hereinafter referred to as second prior art) has been developed. As shown in FIG. 2, the second prior art is based on the first prior art and is configured by forming an aluminum oxide (Al2O3) film 20 on the silicon oxide film 18. Energy of acoustic wave is confined between the piezoelectric substrate 12 and the silicon oxide film 18 and is propagated through the interface therebetween.
Japanese Patent Application Publication No. 52-16146 (Document 2) discloses an acoustic wave device having a structure designed to improve the temperature characteristic. The piezoelectric substrate is made of ceramic, and a titanium oxide (TiO2) film is provided between the comb-like electrodes and the piezoelectric substrate.
Japanese Patent Application Publication No. 11-31942 (Document 3) discloses an electromechanical coupling coefficient of a device having a dielectric material provided below the comb-like electrodes.
The acoustic wave devices of the first and second prior arts have resonance frequencies and anti-resonance frequencies that have mutually different temperature coefficients. The temperature characteristics at the resonance and anti-resonance frequencies can be improved separately. However, it is very difficult to simultaneously improve the temperature characteristics at the resonance and anti-resonance frequencies.